WASHINGTON — With resistance to tougher gun laws stiffening in Congress, a visibly frustrated President Obama on Thursday implored lawmakers and the nation not to lose sight of the horrors of the school massacre in Newtown, Conn.

“The notion that two months or three months after something as horrific as what happened in Newtown happens and we’ve moved on to other things?” Mr. Obama said in remarks at the White House, surrounded by relatives and friends of victims of gun violence, including some from Newtown. “That’s not who we are. That’s not who we are. And I want to make sure every American is listening today.”

The president has just a small window in which to persuade Congress to back a series of gun control measures that will come up for a vote in the Senate early next month. And his remarks, delivered in an impassioned and off-script manner, were aimed at reviving the impetus that gun-control advocates fear they are losing as more time passes since the shootings.

A filibuster threat is growing in the Senate. Senator Harry Reid of Nevada, the majority leader, has said a ban on certain styles of semiautomatic weapons is virtually assured of defeat. And a senior Republican senator who opposes the president’s efforts, Charles E. Grassley of Iowa, is now floating a competing gun bill.

These complications only add to the strain surrounding negotiations over a bipartisan bill that would strengthen the background check system for gun purchases — talks that have so far drawn the support of only one Republican, Mark Kirk of Illinois.

As senators at the heart of those negotiations returned to their home states this week, their staffs continued to try to reach consensus back in Washington. But they have yet to produce anything more than an outline of what legislation might look like.

Mr. Obama’s appearance, from the East Room of the White House, suggested just how delicate the situation had become. Rather than read from teleprompters, he seemed to speak extemporaneously much of the time and expressed irritation in a way that he generally does not. At some moments, he paused and took a breath as if collecting himself and circled back to some of his points for emphasis.

“Shame on us if we’ve forgotten,” he said.“I haven’t forgotten those kids. Shame on us if we’ve forgotten.”

The renewed push by the president, who will travel to Colorado next week to rally support for new gun measures, is just one piece in a broader nationwide effort, timed to coincide with the two-week Congressional recess, by gun control groups like the Brady Campaign to Prevent Gun Violence, and Mayor Michael R. Bloomberg’s coalition.

At the same time, the National Rifle Association is activating its base, ensuring that Congressional offices and town hall meetings over the next week will be swamped with competing agendas on how to combat gun violence.

“What we face right now is the most dire threat to the association and to our freedom,” said Andrew Arulanandam, an N.R.A. spokesman.

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Relatives of victims of gun violence gathered at the White House on Thursday.CreditDoug Mills/The New York Times

Indeed, gun rights activists are being challenged by a highly coordinated and expensive effort to defeat them, not to mention a galvanized group of voters who were outraged by the Newtown shooting and have pledged to volunteer.

The Brady Campaign this week began a campaign to call and e-mail thousands of supporters, urging them to attend more than 150 Congressional town hall meetings, many in Republican-leaning states where Democrats are up for re-election.

“Basically we’re saying, ‘Drop everything. There’s a town hall tonight,’ ” said Brian Malte, the director of mobilization for the Brady Campaign.

People will be equipped with talking points like poll numbers that show 9 out of 10 Americans support universal background checks, including 7 out of 10 N.R.A. members. And they will be encouraged to ask their senators and representatives direct questions like, “Do you support universal background checks?” Mr. Bloomberg’s group, Mayors Against Illegal Guns, said it was convening 120 events across the nation in support of gun measures, including in Columbus, Ohio; Durham, N.C.; and Golden, Colo. The group began a $12 million ad campaign aimed at 15 senators this week.

“Americans want this, and today Americans are making their voices heard,” said the group’s chairman, John Feinblatt.

In at least two crucial cases, senators appear to be listening. Two of the senators who were the subjects of efforts of the mayors’ group, Kay Hagan of North Carolina and Joe Donnelly of Indiana are now signaling their support for expanded background checks.

Other senators are digging in. Five Republican senators have now signed on to a pledge to filibuster “any additional gun restrictions.” They include Mike Lee of Utah, Rand Paul of Kentucky and Ted Cruz of Texas. On Thursday, Marco Rubio of Florida and James Inhofe of Oklahoma announced their intentions to join the filibuster as well.

As the gun control debate played out on Thursday, Connecticut officials unsealed several search warrants itemizing the enormous cache of guns, ammunition, knives and swords that were seized from the home and car of Adam Lanza, who shot and killed 20 first graders and 6 educators in Newtown last December.

David Wheeler, whose 6-year-old son, Benjamin, was killed in the massacre, said in an interview that he and his wife, Francine, hoped the details of the warrant would affect change in gun-control laws and in the communication among law enforcement officials about people who might pose a threat. “We’re starting to get tired of the insanity in Washington and Hartford,” he said. “It’s absolutely despicable, really. I would hope that some of these details will spark enough soul-searching to get something done.”

Details about the items seized at the Lanza home underscored the difficulty gun-control advocates face in Congress. Much of the arsenal would remain legal under even the most far-reaching gun-control measures being considered. But two of the guns Mr. Lanza brought with him to the school, the Bushmaster XM15-E2S he used in the killings, and a Saiga-12 shotgun found in his car, would be outlawed under the assault weapons ban proposed by Senator Dianne Feinstein of California.

Opponents of stronger gun-control laws have long said that new laws would arbitrarily single out certain weapons while ignoring issues like deficiencies in the mental health system. “The president asserts his new gun proposals will reduce violent crime, yet provides no evidence that these or any other law would have prevented tragedies such as Newtown,” Senator Lee said. “While having Congress vote on new gun laws may make the president feel like he’s doing something constructive, the proposals’ primary effect would be to limit the rights of law-abiding citizens.”

Mindful of the fact that passions are rising among gun rights activists as they seem to be ebbing in the other direction, Mr. Obama sought to draw on the emotion and revulsion around the Newtown shooting.

“We need everybody to remember how we felt 100 days ago and make sure that what we said at that time wasn’t just a bunch of platitudes, that we meant it,” he added. To lawmakers, he added sternly, “Don’t get squishy because time has passed and maybe it’s not on the news every single day.”

Michael Cooper contributed reporting from New York, and Elizabeth Maker from Connecticut.

A version of this article appears in print on , on Page A21 of the New York edition with the headline: Months After Massacre, Obama Seeks to Regain Momentum on Gun Laws. Order Reprints | Today’s Paper | Subscribe